1. Field of the Invention
This invention relates to a process for the production of an optical glass article and more particularly, it is concerned with a process for producing a lens material having a three dimensional refractive index distribution inside the lens, which can be used as a lens for a camera, precision optical device or optoelectronic instrument. Furthermore, this invention relates to a rod lens which having a refractive index which varies in the radius direction as a lens of the refractive index distribution type.
2. Description of the Prior Art
Rod lenses of graded refractive index distribution type (which will hereinafter be referred to as "rod lens of graded type") having a parabolic distribution of the refractive index, whose refractive index lowers gradually from the center to the outside in the radius direction, have lately been used increasingly as optical parts in duplicating machines, facsimile devices and optical communications. These rod lenses are made of glass materials or plastic materials and some of them have been put to practical use ("Nikkei Electronics" 1979, 8/20, page 64-74, or "Kogyo Zairyo" 1980, Vol. 20, No. 10, page 85-96). In particular, Selfoc (trade mark) lenses made by the ion exchange method using multicomponent glasses are well known.
However, such an ion exchange method is essentially limited by the diffusion speed of a network modifier ion playing a role of changing the refractive index under such a temperature condition that glass itself is not deformed. Thus, a cation with a high diffusion speed should be used, and, for example, a monovalent cation such as Tl, Cs, Rb, K, Na or Li ion can only be used as the network modifier ion. Therefore, it is not easy to reduce the dispersion of the refractive index. Tl should be used for the purpose of increasing the differences of refractive indexes, but handling Tl is difficult because of its poisonous character. Furthermore, when it is desired to prepare a rod lens with a large diameter, e.g. larger than 3 mm.phi., the ion exchange method is not suitable for practice on a commercial scale, since a long time is taken for ion exchange at a temperature at which glass is not deformed or broken.
In addition, the molecular stuffing method based on another principle has been proposed as a method whereby the above described disadvantages can be overcome. This is a method of making a glass rod comprising precipitating CsNO.sub.3 with a concentration distribution in the micropores of a porous glass prepared through steps of phase separation, leaching out and washing, collapsing this and doping Cs.sub.2 O in such a manner that the concentration of Cs.sub.2 O be in a parabolic distribution from the center to the outside (Japanese Patent Application (OPI) Nos. 28339/1975 (U.S. Pat. Nos. 3,938,974 and 4,313,748), 126207/1976 (U.S. Pat. Nos. 4,110,093, 4,110,096, 4,220,682, 4,236,930 and 4,313,748) and 102324/1978 (U.S. Pat. Nos. 4,183,620, 4,188,198 and 4,299,608)).
However, these methods have the disadvantage that in a porous glass obtained by phase separation, leaching out and washing, there are disorders of the concentration distribution of Cs.sub.2 O and the refractive index distribution due to the disorder of the pore diameter distribution, i.e. abnormal growth of the connected micropores formed in the step of leaching out. A further disadvantage is that since the porosity of such a porous glass is less than 50% and it is difficult to make it larger, the difference of refractive indexes amounts to at most about 2.0% in a glass body prepared by stuffing with a high concentration aqueous solution of CsNO.sub.3, precipitating CsNO.sub.3, unstuffing CsNO.sub.3, further reprecipitating CsNO.sub.3 and collapsing. Accordingly, various efforts have been made to overcome the disadvantages and to develop a porous glass having connected micropores with a desirable and uniform pore diameter distribution.